Polypropylene has been utilized in diverse states for a wide range of raw materials for electric appliances owing to its excellent electrical insulation characteristics. For example, high-purity polypropylene which is extremely reduced in impurities caused by a catalyst is made into a film as thin as 20 μm or less and, in recent years, 5 μm or less, and is applied to a high-performance condenser (for example, Japanese Patent Application Laid-Open (JP-A) No. 6-236709).
However, it is difficult to say that the electrical insulation characteristics of polypropylene currently used are universal in all applications, and if a polypropylene having higher insulation characteristics can be stably provided, it is industrially very useful. As a method for solving this problem, earnest studies have been made to provide a highly purified polypropylene, for example, by using a method where remaining catalyst residue and chlorine content is reduced as much as possible, as disclosed in publications of JP-A Nos. 62-113548 and 2-150443. The present inventors also recognize that the insulation characteristics of polypropylene are significantly improved by these methods. At the same time, the present inventors also recognize that such polypropylene obtained in these methods does not necessarily provide a fixed level of electric insulation characteristics well reproducibly. Namely, the electric insulation characteristics sometimes vary largely between lots and the electric insulation characteristics sometimes vary depending on the position of sampling even if polypropylene materials obtained in the same lot are molded to produce electric materials. It is reasonable that in the case of continuously producing molded products for electric material from polypropylene in an industrial scale, a limitation is placed upon the frequency of pre-quality control of raw material polypropylene from the viewpoint of production speed and production cost. Therefore, if a maximum amount of polypropylene for electric materials can be obtained stably by making pre-quality checks in the smallest frequency, it can be said that this gives very enormous contribution to the development of polypropylene industries.